This invention relates to ceramic thermal banier coatings on substrates, and, more particularly, to aircraft gas turbine components protected by such coatings.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the hot-section components of the engine. These components include the turbine vanes and turbine blades of the gas turbine, upon which the hot combustion gases directly impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1800-2100xc2x0 F. These components are also subject to damage by oxidation and corrosive agents, as well as impact damage and erosion by particles entrained in the combustion gas stream.
Many approaches have been used to increase the operating temperature limit and service lives of the turbine blades and vanes to their current levels, while achieving acceptable oxidation, corrosion, erosion, and impact resistance. The composition and processing of the base materials themselves have been improved. Cooling techniques are used, as for example by providing the component with internal cooling passages through which cooling air is flowed.
In another approach used to protect the hot-section components, some of the surfaces of the turbine blades and vanes are coated with thermal barrier coating systems. The thermal barrier coating systems typically include a bond coat that contacts the substrate, and a ceramic thermal barrier coating (TBC) layer overlying the bond coat. The bond coat protects the articles against the oxidative and corrosive effects of the combustion gas. The ceramic layer provides thermal insulation and some environmental protection. The turbine blades and turbine vanes are thereby able to run cooler and are more resistant to environmental attack in the presence of the thermal barrier coating systems.
Although the thermal barrier coating approach is operable, there is opportunity for improvement. It would be desirable to improve the thermal insulation properties of the ceramic thermal barrier coating, as well as to increase its resistance to impact damage. The present invention fulfills this need, and further provides related advantages.
The present invention provides a structure in which a substrate is protected by an overlying ceramic layer. The ceramic layer has improved insulation properties as compared with prior ceramic layers, as well as improved resistance to impact damage. The ceramic layer of the invention is compatible with the use of bond coats, and in some circumstances dispenses with the need for a bond coat.
A structure comprises a substrate, and a ceramic coating overlying and bonded to the substrate. The ceramic coating comprises an open-cell solid foam of ceramic cell walls having an interconnected intracellular volume therebetween.
Within this broad concept, a number of features and embodiments are particularly preferred. Preferably, the substrate comprises a nickel-base superalloy and is a component of a gas turbine engine. The ceramic cell walls comprise aluminum oxide, but there may be additions of ceramic modifiers to improve specific properties such as impact resistance. The ceramic cell walls preferably exceed about 60 percent by volume, and most preferably comprise from about 60 to about 80 percent by volume, of the foam. A bond coat may be disposed between the substrate and the ceramic coating, to aid in bonding the ceramic coating to the substrate.
Most preferably, the intracellular volume is substantially empty and porous, although it may be filled wholly or partially with a metal. The portion of the ceramic coating which is porous offers improved insulation of the underlying substrate. It also affords increased resistance to impact damage, as it may locally crush and fracture without introducing cracks which propagate for long distances through the ceramic coating and possibly cause the ceramic coating to delaminate.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.